Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Financial assets:
Investment debt securities available-for-sale	$ 3,000.3	$ 2,960.0
Investment debt securities held-to-maturity	72.6	94.5
Derivative assets	65.1	23.2
Financial liabilities:
Derivative liabilities	59.5	22.2
Carrying Amount
Financial assets:
Cash and cash equivalents	869.2	1,076.8
Investment debt securities available-for-sale	3,000.3	2,960.0
Investment debt securities held-to-maturity	70.2	92.3
Accrued interest receivable	46.3	46.7
Mortgage servicing rights, net	27.8	30.2
Loans held for sale	94.4	100.9
Net loans held for investment	8,788.9	8,857.7
Derivative assets	65.1	23.2
Deferred compensation plan assets	17.3	18.2
Total financial assets	12,979.5	13,206.0
Financial liabilities:
Total deposits, excluding time deposits	10,244.5	10,213.5
Time deposits	1,320.6	1,450.0
Securities sold under repurchase agreements	614.1	697.6
Accrued interest payable	10.3	12.1
Long-term debt	13.9	13.9
Subordinated debentures held by subsidiary trusts	86.9	86.9
Derivative liabilities	59.5	22.2
Deferred compensation plan liabilities	17.3	18.2
Total financial liabilities	12,367.1	12,514.4
Estimated Fair Value
Financial assets:
Cash and cash equivalents	869.2	1,076.8
Investment debt securities available-for-sale	3,000.3	2,960.0
Investment debt securities held-to-maturity	72.6	94.5
Accrued interest receivable	46.3	46.7
Mortgage servicing rights, net	27.8	34.8
Loans held for sale	94.4	100.9
Net loans held for investment	8,856.5	8,930.7
Derivative assets	65.1	23.2
Deferred compensation plan assets	17.3	18.2
Total financial assets	13,049.5	13,285.8
Financial liabilities:
Total deposits, excluding time deposits	10,244.5	10,213.5
Time deposits	1,324.9	1,446.6
Securities sold under repurchase agreements	614.1	697.6
Accrued interest payable	10.3	12.1
Long-term debt	10.5	10.4
Subordinated debentures held by subsidiary trusts	81.5	81.3
Derivative liabilities	59.5	22.2
Deferred compensation plan liabilities	17.3	18.2
Total financial liabilities	12,362.6	12,501.9
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	869.2	1,076.8
Investment debt securities available-for-sale	0.0	0.0
Investment debt securities held-to-maturity	0.0	0.0
Accrued interest receivable	0.0	0.0
Mortgage servicing rights, net	0.0	0.0
Loans held for sale	0.0	0.0
Net loans held for investment	0.0	0.0
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	869.2	1,076.8
Financial liabilities:
Total deposits, excluding time deposits	10,244.5	10,213.5
Time deposits	0.0	0.0
Securities sold under repurchase agreements	0.0	0.0
Accrued interest payable	0.0	0.0
Long-term debt	0.0	0.0
Subordinated debentures held by subsidiary trusts	0.0	0.0
Derivative liabilities	0.0	0.0
Deferred compensation plan liabilities	0.0	0.0
Total financial liabilities	10,244.5	10,213.5
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Investment debt securities available-for-sale	3,000.3	2,960.0
Investment debt securities held-to-maturity	72.6	94.5
Accrued interest receivable	46.3	46.7
Mortgage servicing rights, net	27.8	34.8
Loans held for sale	94.4	100.9
Net loans held for investment	8,852.9	8,906.7
Derivative assets	65.1	23.2
Deferred compensation plan assets	17.3	18.2
Total financial assets	12,176.7	12,185.0
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	1,324.9	1,446.6
Securities sold under repurchase agreements	614.1	697.6
Accrued interest payable	10.3	12.1
Long-term debt	10.5	10.4
Subordinated debentures held by subsidiary trusts	81.5	81.3
Derivative liabilities	59.5	22.2
Deferred compensation plan liabilities	17.3	18.2
Total financial liabilities	2,118.1	2,288.4
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Investment debt securities available-for-sale	0.0	0.0
Investment debt securities held-to-maturity	0.0	0.0
Accrued interest receivable	0.0	0.0
Mortgage servicing rights, net	0.0	0.0
Loans held for sale	0.0	0.0
Net loans held for investment	3.6	24.0
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	3.6	24.0
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	0.0	0.0
Securities sold under repurchase agreements	0.0	0.0
Accrued interest payable	0.0	0.0
Long-term debt	0.0	0.0
Subordinated debentures held by subsidiary trusts	0.0	0.0
Derivative liabilities	0.0	0.0
Deferred compensation plan liabilities	0.0	0.0
Total financial liabilities	$ 0.0	$ 0.0